King turned professional in 1991 and advanced steadily through the rankings, reaching the top 48 by 1996. His Welsh Open final run lead to a top 32 place in the end of season rankings, and he continued to progress by reaching the top 16 a year later. He made his debut at the World Snooker Championship in 1994, losing to Darren Morgan in the first round.

He remained in the top 16 the following season without reaching a ranking quarter-final, but dropped out a year later. He made an immediate return in 2000/2001, climbing to a career-high of #11. A poor 2002/2003 season ended with him dropping out of the top 16 again after defeat to Drew Henry 10–5 in the first round of the World Championship. In the post-match interview, King suggested that he had lost patience with the game and would quit because "I've just had enough of the game."[4]

After the 2004 World Championship, King became involved in a tense battle with Quinten Hann after his friend Andy Hicks eliminated Hann in a controversial first-round clash. Hann had been making provocative gestures during the match and after Hicks took victory, the two players had an altercation, with an angry Hann squaring up to Hicks. Upset by Hann's behaviour, King challenged the Australian to a boxing match,[5] for which King was later criticised, as Hann's behaviour had already put the game into disrepute. Nevertheless, the bout went ahead and Hann controversially won the fight on a points decision.

In December 2004, King beat Robin Hull, Mark Williams, Alan McManus and John Parrott en route to the semi-finals of the UK Championship, losing 9–4 to eventual winner Stephen Maguire. At 8–3, a mobile phone went off in the crowd, prompting King to quip "if that's my missus, tell her I'll be home soon". In 2005, King beat Ronnie O'Sullivan 9–8 at the same event, having fought back from 5–3 down in a match which saw his opponent sitting with a wet towel draped over his head when King was at the table. King would eventually lose 9–6 in the last 16 to Joe Perry, but made a career best 146 break at the 2006 Championships.

Defeat to Neil Robertson (he was trailing 7–2 but only lost 10–9) in the qualifying for the World Championship in 2005 cost him the chance of a top 16 return, but King qualified a year later, resiliently losing 10–6 to Stephen Maguire in the first round. In 2007, King went out to David Gilbert in the qualifying round for the event.

In the 2008 World Championship, King beat six-times runner-up Jimmy White to qualify,[6] then shocked the previous year's runner-up Mark Selby 10–8, having trailed 5–3 overnight. After potting the match ball, King showed his delight by shouting "Get in".[7] He lost to Peter Ebdon in the last 16, but his top-16 place was secure and he would start the 08/09 campaign ranked 15th.[8]

At the 2009 Grand Prix King needed three snookers in the deciding frame against Ricky Walden to claim a 5–4 victory. He managed to get them and won by potting the final black. In the next round he lost to Robert Milkins.

At the 2010 China Open King defeated Thai player James Wattana 5–4 in the first round after trailing 2–4. He then won his second match 5–3 against Chinese player Tian Pengfei to reach his first Quarter final since the 2007 Malta Cup. A double triumph for King, the win helped to maintain his Top 16 place for the 2010/11 season.

At the 2010 World championship, King lost his first round match against former 6 times World Champion Steve Davis by 9–10. In the deciding frame, Davis lead 55–37, with only 13 points available. King's attempts at getting the snooker he needed to win ended with him fluking the pink in the pocket.[9]

King made an impressive start to the new season when he reached the Quarter Final of the 2010 Shanghai Masters. King was too strong for qualifier Joe Delaney in the first round, winning 5–3. The same scoreline saw him overcome Peter Ebdon in the next round but King was then beaten comfortably by Mark Selby 5–1. However his lack of form in the following ranking events made him fall out of the top 16 after the 2011 Welsh Open.

At the 2011 Masters, King beat defending champion Mark Selby 6–4 to reach his first quarter-final at the event since 1999. After the match, King suggested that his comeback from 0–2 down was prompted by his opponent's celebration to winning the second frame on the black, when he "put his cue up and it gave me the hump a bit" [10] King then lost in the quarter-finals 1–6 against Jamie Cope.[11] He ended the season by losing 7–10 to Graeme Dott in the World Championship and was ranked 26, dropping 11 places during the year.[12] He was therefore out of the top 16 in the end of season rankings for the first time since the 2007/2008 season.[13]